1. Field of the Invention:
The present invention relates to an oil pressure control means for an automatic transmission for automobiles and, more particularly, an improvement of such an oil pressure control means with regard to the speed shift performance when the engine is in a cold state before warming up.
2. Description of the Prior Art:
In the automatic transmission comprising a fluid torque converter and a reduction gear means including a reduction gear mechanism and a plurality of friction engaging means, the operation of the friction engaging means is changed over in various manner in accordance with the variation of the running condition of the vehicle to constantly establish the most desirable speed shift stage for the running condition of the vehicle. The changing over control of the friction engaging means is effected by an oil pressure control means which incorporates speed shift valves each being adapted to shift between two shift positions depending upon the balance of the throttle pressure which varies in accordance with the amount the accelerating pedal is pushed or the opening of the throttle valve and the governor pressure which varies in accordance with the vehicle speed so that the reduction gear means is set at a selected desirable speed stage depending upon the balance of the throttle pressure and the governor pressure, i.e. the balance of the amount the accelerating pedal is pushed and the vehicle speed. Therefore, even when the accelerating pedal is pushed for a constant amount, if the vehicle speed at the instant is relatively low, the throttle pressure is relatively higher than the governor pressure in view of the contrast therebetween, whereby the reduction gear means is set at a lower speed stage, whereas if the vehicle speed is relatively high, the throttle pressure is relatively lower than the governor pressure in view of the contrast therebetween, whereby the reduction gear means is set at a higher speed stage. In the conventional automatic transmission wherein the speed stage of the reduction gear means is selected depending upon the balance or contrast of the throttle pressure and the governor pressure, it happens that the reduction gear means is set at a higher speed stage even when the vehicle speed is relatively low, if the amount the accelerating pedal is pushed is also so small that the throttle pressure is also sufficiently low relative to the low governor pressure at the relatively low vehicle speed. Particularly when an overdrive means is incorporated in the automatic transmission, it happens that the transmission operates in the overdrive condition even in a low speed operation of the vehicle.
It does not cause any serious problem that the vehicle is operated at a low speed in a high speed gear such as the overdrive stage if the engine is in the warmed up condition. However, if the engine is operated in a high speed gear before it is warmed up, there occur problems such as the increase of the fuel consumption and the increase of the emission of noxious components, particularly NOx in the exhaust gases.